Iron-based brazing filler metals

ABSTRACT

A plurality of parts is brazed using an iron-based brazing filler metal. The parts generally include stainless steel, and the brazed assembly forms a heat exchanger characterized by effective corrosion resistance and low rates of leaching of nickel into fluids passing therethrough. The heat exchanger is especially suited for use in processing items intended to be ingested by humans or animals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to brazing of metal parts, and moreparticularly, to a homogeneous, ductile iron-based brazing materialuseful in brazing stainless steels, and a method for brazing stainlesssteel components to form articles of manufacture, wherein the brazedstainless steel components reduce the propensity of nickel to leach fromsuch articles in water.

2. Description of the Prior Art

Brazing is a process for joining metal parts, often of dissimilarcomposition, to each other. Typically, a filler metal that has a meltingpoint lower than that of the metal parts to be joined together isinterposed between the metal parts to form an assembly. The assembly isthen heated to a temperature sufficient to melt the filler metal. Uponcooling, a strong, leak-tight joint is formed. The assembled parts mayeither constitute a finished article of manufacture or may form asub-component for use in a further manufacturing operation.

The selection of a particular brazing filler metal for a specificapplication depends on a variety of factors, including requirementsrelated to the components to be joined and to the conditions under whichthe assembly ultimately must operate.

One basic consideration is temperature. Brazing filler metals arecharacterized by their solidus and liquidus temperatures. The term“solidus” refers to the highest temperature at which a metal or alloy iscompletely solid, and the term “liquidus” refers to the lowesttemperature at which the metal or alloy is completely liquid. In anybrazing process, the brazing filler metal must possess a solidustemperature that is high enough to provide the brazed assembly withadequate integrity to meet the desired service requirements and yet havea liquidus that is low enough to be compatible with the temperaturecapabilities of the parts being joined.

Another consideration is corrosion resistance. Many brazed assembliesmust operate under environmental conditions that are conducive tocorrosion, especially in the vicinity of the brazement. The propensityof a given system to corrode is strongly influenced by the gases orliquids to which the system is exposed and by typical operatingtemperatures.

One class of devices which are frequently assembled using brazing as ajoining technique is heat exchangers. These devices are known in avariety of configurations. Generally stated, heat exchangers allow heatto be transferred across an interface that separates one circulatingfluid from another circulating fluid. It is generally essential that thefluids, either of which can be gaseous or liquid, be kept separate.Hence, it is critical that brazed joints which define, at least in part,the interface maintain structural integrity under a full range ofoperating conditions and for a prolonged service life.

One field of use wherein heat exchangers find utility is in theprocessing of materials which are ultimately intended for humaningestion and consumption. These include foodstuffs, as well as fluidssuch as water, beverages, juices, and the like. The metallic materialsused for the construction of heat exchangers appointed for suchapplications are of critical importance. Such metallic materials notonly need to provide excellent operative characteristics with regard toheat transfer, but also must be compatible with the substances to whichthey are exposed. One particular concern is the requirement that therebe no undesired leaching or desolution of any elemental or molecularcomponent species of the materials of construction that is harmful oradds undesirable taste to the fluids. If a harmful species or anundesirable taste is present, then it is imperative that any leaching ofcausative materials be minimized. Frequently, local governmental orregulatory authorities have established maximum amounts of materials,such as metal ions, which may be permitted to leach into fluids passingtherethrough. The standard is ordinarily expressed as a maximum amountof leachate that may be present per unit volume of the fluid processed.Ideally, the materials incorporated in heat exchangers (includingbrazing filler metals) and the associated manufacturing methods resultin a device that meets or exceeds applicable regulatory standards underforeseeable operating conditions.

Heat exchangers of the “shell-and-tube,” “plate/plate,” and “plate/fin”types are most usually encountered. In the first configuration, a largerdiameter housing typically referred to as a “shell” encompasses one ormore small diameter tubes or pipes. According to this configuration, afirst fluid (i.e., liquid, gas) passes through the shell and about theexterior of the tubes while simultaneously, a second fluid (liquid, gas)passes through the interior of the tubes. While no physical contact ispermitted between the first and second fluids, heat transfer occursacross the walls of the tubes from the hotter fluid to the cooler fluid.In plate/plate and plate/fin type heat exchangers, again a physicalmember, namely one or more plates separate a first fluid from a secondfluid while heat transfer occurs across the plate. In these types ofheat exchanger (as well as in other assemblies), metals are mostcommonly used due to their high strength and effective heat transfercharacteristics. Typically, the individual parts, which are used to makeup such types of heat exchangers, are joined by brazing. It isimperative that the heat exchanger maintain a physical integrity, andretain the isolation of the fluids from each other and the outsideworld. In addition, the heat exchanger and the joints that secure theinternal components must be resistant to any potential detrimentaleffects which might result from contact with one or both of the fluids.

To minimize such undesired effects, the materials of construction forheat exchangers, particularly those used for foodstuffs, need to be verycarefully selected. Stainless steels, which contain up to about 20%nickel, are very commonly utilized, for stainless steels exhibitdesirable properties, including low leaching rates into fluids or gases,and generally effective corrosion resistance. However, brazingmanufacturing processes carried out at high temperatures may alsoadversely affect the propensity of the stainless steels to leach.Previously, elemental copper was used as a brazing filler metal sincecopper featured low leaching of nickel into fluids, especially water.However, the corrosion resistance of heat exchangers having componentsbrazed using copper as the brazing filler metal is poor. Typically, suchheat exchangers required frequent replacement, resulting in significantcosts for the replacement device and the associated labor, as well aseconomic losses resulting from manufacturing downtime. To improvecorrosion resistance, it was recently found that brazing filler metalswith compositions based primarily on nickel and chromium could beemployed to join stainless steel parts used in such assemblies.Unfortunately, it was also found that when such nickel-based brazingfiller metals were used, an undesirably high amount of nickel oftenleached into water or other fluids flowing through such assemblies.

Inasmuch as such nickel-based brazing filler metals include asignificant proportion of nickel, nickel-based brazing filler metals arebelieved to be the source of the undesired nickel leachate. For thisreason, use of nickel-based brazing filler metals should be avoided inapplications where nickel leaching into a fluid presents a concern, asis the case when materials passing through the heat exchangers are to beused for human ingestion or consumption. Not surprisingly, governmentalregulations in some countries have imposed strict limitations on theamount of nickel which may be leached into fluids for human ingestion orconsumption. It is to one or more of these technical needs that thepresent invention is directed.

SUMMARY OF THE INVENTION

The present invention provides a method to fabricate heat exchangers andother articles of manufacture by brazing components thereof with aniron-based brazing filler metal composition. Brazed assembliesadvantageously exhibit effective general corrosion resistance and lowrates of leaching of nickel into fluids passing through either side ofthe heat exchanger. As a result, the heat exchanger is highly suited forexposure to items intended for ingestion by humans or animals.

In a first aspect, the present invention provides a method tomanufacture assemblies, especially assemblies which include partscomprising stainless steels. Such assemblies comprise parts joined usingiron-based brazing filler metals. When manufactured, the assemblies arecharacterized by general corrosion resistance and by low leaching ratesof nickel. The method comprises: juxtaposing at least two parts todefine one or more joints therebetween; supplying to the one or morejoints an iron-based brazing filler metal composition that is a ductile,amorphous brazing foil; heating the juxtaposed parts and the brazingfiller metal to cause melting of the iron-based brazing filler metal;and cooling the melted iron-based brazing filler metal to produce abrazed joint that minimizes an amount of nickel leaching into a fluidcontacting the brazed joint. The heating and cooling operations aregenerally carried out either in a protective gas atmosphere or in avacuum.

In a second aspect, an iron-based brazing filler metal alloy is used.Typically, the iron-based brazing filler metal having essentially acomposition with the formula Fe_(a)Cr_(b)B_(c)Si_(d)X_(e), wherein X ismolybdenum or tungsten and incidental impurities, wherein the subscripts“a”, “b”, “c”, “d”, “e” are all in atom percent, and wherein “b” isbetween about 0 and 5, “c” is between about 10 and about 17, “d” isbetween about 4 and about 10, “e” is between about 0 and about 5, and asum “a”+“b”+“c”+“d”+“e” is approximately equal to 100.

The iron-based brazing filler metal is especially suited for fabricatingheat exchangers and other assemblies of the invention which require lownickel leaching rates. Generally, the iron-based brazing filler metal isprepared in the form of a homogeneous, ductile ribbon or strip.

The alloys of the present invention include substantial amounts of boronand silicon, which are present in the crystalline solid state in theform of hard and brittle borides and silicides. Accordingly, the alloysof the invention are particularly suited for fabrication into flexiblethin foil by rapid solidification techniques. Foil produced in such amanner is a metastable material having at least a 50% glassy structureand a thickness ranging from about 18-50 μm (approximately 0.0007 to0.002 inches). Use of a thin flexible and homogeneous foil as a fillermetal is especially beneficial for brazements wherein the matingsurfaces have wide areas with narrow clearances and for brazing jointshaving complex shapes. The alloys of the present invention may also beproduced in powder form by gas or water atomization of the alloy or bymechanical comminution of a foil comprised thereof. Other methods, suchas rolling, casting, and other powder metallurgical techniques may bealso be used to prepare such alloys.

Further aspects and features of the invention will become more apparentfrom the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood and further advantages willbecome apparent when reference is had to the following detaileddescription of the various embodiments of the invention and theaccompanying drawings, wherein like reference numerals denote similarelements throughout the several views and in which:

FIG. 1 is a perspective view of a portion of a shell-and-tube heatexchanger in a partially disassembled state, along with a brazing foilpreform adapted for use in brazing the components of the heat exchangerin accordance with an embodiment of the present invention;

FIG. 2 is a cross-sectional view of a heat exchanger of the plate andfin type brazed using iron-based brazing filler metal in accordance withan embodiment of the present invention; and

FIG. 3 is a cross-sectional view of a heat exchanger of the plate-platetype brazed using iron-based brazing filler metal in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present invention is directed to methods to manufacture assemblieswhich include brazed metal components, wherein the manufacturedassemblies are advantageously characterized by low leaching rates ofnickel into fluids which flow through the manufactured assembly andgeneral corrosion resistance. The invention further provides aniron-based brazing filler metal suitable for such a manufacturingprocess.

In accordance with the present invention, an iron-based brazing fillermetal alloy is used. Generally, the iron-based brazing filler metal hasessentially a composition with the formula Fe_(a)Cr_(b)B_(c)Si_(d)X_(e),wherein X is molybdenum or tungsten, and incidental impurities, whereinthe subscripts “a”, “b”, “c”, “d”, “e” are all in atom percent, andwherein “b” is between about 0 and 5, “c” is between about 10 and about17, “d” is between about 4 and about 10, “e” is between about 0 andabout 5, and a sum “a”+“b”+“c”+“d”+“e” is approximately equal to 100.

In any brazing process, the brazing filler metal must have a meltingpoint high enough to provide joint strength meeting service requirementsof the brazed metal parts. Too high a melting point may weaken orsensitize the base metal. Additionally, too high a melting point mayerode the base metal in the vicinity of the joint region. A fillermaterial must also be compatible, both chemically and metallurgically,with the materials being brazed.

Iron-based brazing filler metals particularly useful in the methods andassemblies of the present invention are metal alloys which may beproduced in various forms, including, but not limited to, powders,foils, ribbons and wires, according to well known techniques. Methodscommonly used to fabricate alloys in powder form include gas or wateratomization, as well as mechanical pulverization. Alloys of the presentinvention are most generally formed into ductile foils, ribbons or wireby rapid solidification. Production of metal alloys by rapidsolidification typically entails quenching a melt of the requisitecomposition by rapidly cooling at a rate of at least about 10³° C./sec,although higher rates are known and more commonly used. Of the rapidsolidification processes available today, the most typical processemploys a rapidly rotating chill wheel onto which a molten alloy isimpinged and cast into wide ribbon. One such process suitable for themanufacture of the brazing filler metal of the present invention aswide, ductile ribbon is disclosed by U.S. Pat. No. 4,221,257.

Ideally, the iron-based brazing filler metal of the present invention isin the form of a ductile foil which may be readily handled. In such aform, the iron-based brazing filler metal of the present invention isconveniently prepared in a variety of shapes that conform to contoursused in the assembly of complex part assemblies. Formation into complexshapes may occur by bending or stamping the ductile foil.

Generally, the brazing foil of the invention is essentially homogeneousin composition, that is to say, that it includes no binders, such asorganic binders which would provide the potential for void formation orthe deposition of contaminating residues during brazing. The homogeneouscomposition of the foil results in liquidus and solidus temperaturesthat are uniform throughout, further promoting uniform melting and theformation of a strong, uniform, void-free brazed joint.

Rapidly solidified products produced from homogeneous melts of thealloys are usually homogeneous in the solid state. The products may beglassy or crystalline, depending upon the alloy compositions andprocessing parameters. In addition, products that are at least 50%glassy usually exhibit sufficient ductility to enable foil, ribbon andwire forms of the alloys to be bent to a radius as small as ten times athickness of the foil, ribbon or wire without fracture. Typically, theiron-based brazing filler metals of the present invention are metalalloys which are formed by rapidly solidifying a melt of the metal alloyat quenching rates of at least about 10⁵° C./sec. Such quenching ratestypically produce alloys which are at least about 50% glassy and, as aresult, are sufficiently ductile so as to enable the alloys to bestamped into complex shapes. More typically, the alloys of the presentinvention are at least about 80% glassy. Most typically, the alloys aresubstantially fully glassy (i.e., at least 90% glassy), and thus exhibita significantly elevated degree of ductility.

The alloys provided by the present invention are particularly suited foruse as brazing filler metals in the methods described herein. Mostgenerally, the alloys are produced in foil form and are usefulregardless of whether the foil is glassy or microcrystalline.Alternatively, the alloys may be prepared in the form of a foil with acrystalline solid solution or glassy metal structure, and in both cases,may be heat treated to obtain therein a fine-grained crystallinestructure that promotes longer die life when stamping of complex shapesis contemplated. The foils of the present invention typically arebetween about 18 to 50 micrometers (about 0.0007 inches and about 0.002inches) thick. In many instances, the foil thickness correspondsapproximately to the desired gap between parts to be brazed.

The brazing filler metals of the present invention are particularlyuseful for the joining of metal parts, and particularly, stainless steelparts. Stainless steels are most frequently used in processing offluids, including foodstuffs such as juices or other beverages, such aswater, which are ultimately intended for human consumption. Exemplarygrades of such stainless steels include: steel S31603 according to UNSClassifications, as well as type 316L stainless steel, which isdescribed as typically having approximately 0.03 wt. % carbon, 2.00 wt.% manganese, 1.0 wt. % silicon, 10 to 14 wt. % nickel, 16 to 18 wt. %chromium, 2 to 3 wt. % molybdenum, 0.1 wt. % nitrogen and iron as thebalance to 100 wt. %. It is contemplated that other materials benefitingfrom the teaching herein may also be used in accordance with theinvention to afford reduced nickel leaching rates and increasedcorrosion resistance. By way of non-limiting example, such materialsinclude other grades of stainless steel, as well as other corrosionresistant alloys, such as those including nickel.

Most typical brazing filler metals of the present invention include aniron-based brazing filler metal alloy that comprises essentially acomposition with the formula Fe_(a)Cr_(b)B_(c)Si_(d)X_(e), wherein X ismolybdenum or tungsten, and incidental impurities, wherein thesubscripts “a”, “b”, “c”, “d”, “e” are all in atom percent and, andwherein “b” is between about 0 and 5, “c” is between about 10 and about17, “d” is between about 4 and about 10, “e” is between about 0 andabout 5, and a sum “a”+“b”+“c”+“d”+“e” is approximately equal to 100.

The typical brazing filler metal is readily quenched into asignificantly ductile metal strip and exhibits low liquidus temperaturesthat are generally below liquidus temperatures of materials to bebrazed. Moreover, heat exchangers and other similar assemblies brazedusing the typical brazing filler metal are characterized by rates ofnickel leaching that are advantageously lower than average leachingrates and by general corrosion resistance.

In another aspect of the invention, a brazing method is used tomanufacture devices such as heat exchangers and other equipmentcomprising brazed parts. The devices are selected to process materialsfor human ingestion or human consumption and are characterized byreduced leaching rates of nickel into fluids in contact with thedevices. The method includes the operations of: juxtaposing at least twoparts to define one or more joints therebetween; supplying to the one ormore joints an iron-based brazing filler metal in the form of a ductile,amorphous brazing foil; heating the juxtaposed parts and the brazingfiller metal to cause melting of the brazing filler metal; and coolingthe melted brazing filler metal to produce at least one brazed jointthat minimizes an amount of nickel leaching into a fluid contacting thebrazed joint.

Referring now to FIG. 1, a partially disassembled state of a portion ofa heat exchanger 80 of conventional shell-and-tube form is illustrated.The heat exchanger 80 comprises a shell 82 and a plurality of tubes 84,each having an end 86 extending through a suitably dimensioned passagethrough a plate 90. In operation, one fluids flows through tubes, whileanother fluid flows through the inside portion of shell 82 not occupiedby tubes 84. Heat is exchanged in a conventional manner across theinterface defined by the combined external surface area of tubes 84located within shell 82. The diameter of the plate 90 is selected to fitwithin the inside diameter of the shell 82. Edge 92 of plate 90 isgenerally formed with a slight taper to facilitate insertion into shell82 during assembly. It is also contemplated that the outer diameter ofthe plate 90 should have a small clearance relative to the innerdiameter of the shell 82. This clearance between the shell 82 and theplate 90 is usually at least slightly larger than the thickness of thebrazing foil preform 10. The reason for the clearance is that it iscontemplated that the tabs 14 depending from the major planar face 18 ofthe preform 10 are placed in contact with the edge 92 of the plate 90prior to the brazing operation. Similarly, the perforations 16 presentand passing through the planar face 18 are also selected and arranged tocoincide with the placement and dimensions of the ends of the tubes 86.

In one aspect, the assembly of heat exchanger 80 comprises theoperations of positioning brazing foil preform 10 against a primary face94 of plate 90; folding tabs 14 to contact or at least to extendalongside the tapered edge 92; and orienting preform 10 such thatperforations 16 correspond suitably with positioned tube ends 86 and theplate 90. Thereafter, the assemblage is inserted into the shell 82; andthe assemblage is brazed in accordance with specific requirementsnecessary for the materials of construction of the assembly, and withregard to the iron-based brazing filler metal of which the brazing foilpreform 10 is composed.

FIG. 2 illustrates a heat exchanger 15 of a plate-fin type, comprising aplurality of plates 1 and fins 2. Assembly of the heat exchanger 15comprises the operations of: preparing a requisite number of preforms,each being a preselected sheet of brazing filler metal of a preselectedsize comprising an iron-based brazing filler metal; and disposing apreform between each of the adjacent fins and plates to be joined bybrazing. The assemblage is then brazed in accordance with specificrequirements necessary for the materials of construction of theassembly, and with regard to the iron-based brazing filler metalcomprising the brazing preform. After completion of the brazingoperation, a fillet 4 of the brazing filler metal is present insubstantially a full area of contact between adjacent plates 1 and fins2.

FIG. 3 depicts a heat exchanger 25 of a plate-plate type, comprising aplurality of plates 1. The assembly of heat exchanger 25 comprises theoperations of: preparing a predetermined number of preforms, each havinga preselected size of a sheet of brazing filler metal comprising aniron-based brazing filler metal; and disposing a preform between each ofthe adjacent plates 1 to be joined by brazing. This assemblage is thenbrazed in accordance with specific requirements necessary for thematerials of construction of the assembly, and with regard to theiron-based brazing filler metal which comprises the brazing preform.After completion of the brazing operation, a fillet 4 of the brazingfiller metal is present in substantially a full area of contact betweenadjacent plates 1.

In a typical embodiment of the method described above, the iron-basedbrazing filler metal comprises essentially a composition with theformula Fe_(a)Cr_(b)B_(c)Si_(d)X_(e), wherein X is molybdenum ortungsten, and incidental impurities, wherein the subscripts “a”, “b”,“c”, “d”, “e” are all in atom percent, and wherein “b” is between about0 and 5, “c” is between about 10 and about 17, “d” is between about 4and about 10, “e” is between about 0 and about 5, and a sum“a”+“b”+“c”+“d”+“e” is approximately equal to 100.

Typically, in the process described above, the heating and cooling ofthe juxtaposed parts to cause the brazing thereof occurs in a closedoven in a presence of a protective gas such as argon, helium, ornitrogen. Alternately, heating and cooling may occur in a closed ovenunder vacuum conditions as well, and in certain instances, suchconditions are typical. The brazing conditions are typically used inindustry to achieve a high joint strength and integrity when usingfiller metals containing oxygen-active elements such as boron, silicon,and phosphorus.

Manufactured assemblies, and especially heat exchangers manufacturedaccording to the methods described herein are characterized by reducedleaching rates of nickel into water-based fluids passed therethroughwhen compared to assemblies manufactured in accordance with knownmethods comprising brazing using nickel and nickel-chromium based fillermetals. While it is to be understood that any reduction in nickelleaching, particularly into a liquid such as water is to be consideredto fall within the scope of the present invention, a reduction on theorder of at least 50%, typically at least about 70%, and most typicallya reduction of at least about 85%, is attained. Such percentages arebased upon a comparison of nickel leaching rates under identical testconditions of two identical heat exchangers (or other manufacturedassembly) which have been similarly manufactured, but wherein one ismanufactured in a process which includes the use of an iron-based brazefiller metal and an optional, post-brazing conditioning operationdescribed herein, and the other is manufactured conventionally, such asusing a nickel or nickel/chromium-based braze filler metal. For example,optional post-brazing conditioning may include annealing by heating to atemperature below solidus for a predetermined period of time to improvemicrostructure and associated ductivity of a joint.

It is widely understood that corrosion of metallic parts is aconsequence of galvanic action. Corrosion is manifested in a variety offorms of degradation. Corrosion may occur over a large part of a givensurface, or may be localized in a region, such as a region in and arounda brazement. Advantageously, the manufactured assemblies and heatexchangers of the present invention exhibit effective general corrosionresistance. That is, such assemblies are resistant to a wide variety oflocalized and generalized manifestations of corrosion, includinggeneralized or localized removal of material, surface oxidation,rusting, pitting, and the like. The particular mechanism that operatesin a given situation depends on the materials used in constructing anassembly, the materials to which the assembly is exposed, and a time,temperature, and duration of that exposure. In particular, manufacturedassemblies, and especially heat exchangers constructed according to themethods described herein are characterized by superior resistance tocorrosion in water-based fluids when compared to assemblies constructedusing copper-based filler metals. While it is to be understood that anyimprovement in resistance to corrosion is to be considered to fallwithin the scope of the present invention, a reduction on the order ofat least 20%, typically at least about 40%, and most typically areduction of at least about 60%, is attained. Such percentages are basedupon a comparison of the corrosion rates under identical test conditionsof two identical heat exchangers (or other manufactured assembly) whichhave been similarly manufactured, but wherein one is manufactured in aprocess which includes the use of an iron-based brazing filler metal.The general corrosion resistance of the heat exchangers and otherassemblies produced according to the process described hereinadvantageously leads to a substantially longer expected service life ofthe assembly. The increased service life not only lessens the risk offailure during operation, but also reduces the expected frequency ofreplacement or maintenance of such heat exchangers and other assembliesand the attendant disruption of service.

Significant fields of application, wherein the inventive heat exchangersand other assemblies manufactured according to the methods describedherein, include the cooling of drinking water or other beverages. Ofcourse, the methods described herein may be used in manufacture of otherdevices or articles useful both within the technical area related to infood and beverage processing, as well as outside of such a technicalarea.

It will be understood that the present invention has utility not only inthe manufacture of heat exchangers, but also in any application where itis desired to reduce an amount of nickel which may leach from anassembly comprising brazed metal parts and to maintain an advantageouseffective general corrosion resistance. More generally, the inventionadditionally relates to a process for joining two or more metal parts,and particularly two or more stainless steel parts, comprising theoperations of: juxtaposing the at least two parts to define one or morejoints therebetween; supplying to the one or more joints an iron-basedbrazing filler metal having a melting temperature less than that of anyof the parts and having a composition that is a ductile, amorphousbrazing foil; heating the juxtaposed parts and the brazing filler metalto cause melting of the brazing filler metal; and cooling the meltedbrazing filler metal to produce at least one brazed joint that minimizesan amount of nickel leaching into a fluid contacting the brazed joint.

In a typical embodiment of the method of joining parts, the iron-basedbrazing filler comprises essentially a composition with the formulaFe_(a)Cr_(b)B_(c)Si_(d)X_(e), wherein X is molybdenum or tungsten, andincidental impurities, wherein the subscripts “a”, “b”, “c”, “d”, “e”are all in atom percent, and wherein “b” is between about 0 and 5, “c”is between about 10 and about 17, “d” is between about 4 and about 10,“e” is between about 0 and about 5, and a sum “a”+“b”+“c”+“d”+“e” isapproximately equal to 100.

The following examples are presented to provide a more completeunderstanding of the invention. The specific techniques, conditions,materials, proportions and reported data set forth to illustrate theprinciples and practice of the invention are exemplary and should not beconstrued as limiting the scope of the invention.

EXAMPLE 1

Preparation of Iron-based Brazing Filler Metal Strip

Strips of about 2.5 to 25 mm (about 0.10 to 1.00 inch) width and about18 to 50 μm (about 0.0007 to 0.002 inch) thick are formed by squirting amelt of a preselected composition, such as the composition with theformula Fe_(a)Cr_(b)B_(c)Si_(d)X_(e) noted above, by overpressure ofargon onto a rapidly rotating copper chill wheel (surface speed about3000 to 6000 ft/min.). Metastable, ductile, homogeneous ribbons ofsubstantially glassy alloys comprising essentially the compositions(atom percent) set forth in Table I are produced, wherein “bal.” foriron indicates a balance (100% minus the values stated for othercomponents of the composition). TABLE I Fe Cr Co Ni Mo W B Si alloy 1bal. 2.0 15 6 alloy 2 bal. 11 9 alloy 3 bal. 10 10 alloy 4 bal. 15 10alloy 5 bal. 12 10

Example 2

Characterization of Iron-based Brazing Filler Metal Strip

The liquidus and solidus temperatures of selected ribbons havingcompositions set forth in Table I are determined by a DifferentialThermal Analysis (DTA) Technique. The individual samples are heated sideby side with an inert reference material at a uniform rate, and atemperature difference between the individual sample and the inertreference material is measured as a function of temperature. A resultingcurve, conventionally known as a thermogram, is a plot of relativechanges in the temperatures of the sample and the reference materialsduring simultaneous heating vs. temperature, from which the beginning ofmelting and end of melting, which represent the solidus and liquidustemperatures, respectively, are determined. Values thus determined areset forth in Table II below. TABLE II Solidus Liquidus alloy 1 1174° C.(2145° F.) 1182° C. (2157° F.) alloy 2 1138° C. (2080° F.) 1168° C.(2134° F.) alloy 3 1151° C. (2104° F.) 1162° C. (2124° F.) alloy 4 1042°C. (1876° F.) 1148° C. (2098° F.) alloy 5 1092° C. (1998° F.) 1156° C.(2113° F.)

Alloys of the present invention, with solidus and liquidus temperatureslisted in Table II, may be used as filler metals to braze stainlesssteels. Such alloys will melt and flow at temperatures that will notdamage the stainless-steel-based metal parts and, at the same time, areconvenient for industrial brazing processing. As may be seen from TableII, the amounts of boron and silicon may be varied to adjust theliquidus and solidus temperatures of the brazing filler metal materialto desired liquidus and solidus temperatures.

Although a few embodiments of the present invention have been shown anddescribed, it would be appreciated by those skilled in the art thatchanges may be made in these embodiments without departing from theprinciples and spirit of the invention, the scope of which is defined inthe claims and their equivalents.

1. A brazing filler metal consisting essentially of a composition with aformula Fe_(a)Cr_(b)B_(c)Si_(d)X_(e), wherein X is molybdenum, tungsten,or a combination of molybdenum and tungsten, and incidental impurities,wherein the subscripts “a”, “b”, “c”, “d”, “e” are all in atom percent,and wherein “b” is between about 0 and 5, “c” is between about 10 andabout 17, “d” is between about 4 and about 10, “e” is between about 0and about 5, and a sum “a”+“b”+“c”+“d”+“e” is approximately equal to100.
 2. The brazing filler metal as recited by claim 1, said metal beingin a form of one of: a homogeneous, ductile ribbon; a powder; a foil; awire; or a preform.
 3. A brazing filler metal material for joiningobjects by brazing, characterized in that the brazing material consistsof an alloy which contains at least 63% iron and includes 0-5% chromium,10-17% boron, 4-10% silicon, and 0-5% X, wherein X is molybdenum,tungsten, or a combination of molybdenum and tungsten, and incidentalimpurities, all stated in weight percent, and wherein amounts of boronand silicon are varied to adjust liquidus and solidus temperatures ofthe brazing filler metal material to desired liquidus and solidustemperatures.
 4. The brazing filler metal as recited by claim 3, whereinthe brazing filler metal material is in a form of one of: a homogeneous,ductile ribbon; a powder; a foil; a wire; or a preform.
 5. A brazingfiller metal foil for joining objects by brazing, characterized in thatthe brazing foil consists of an alloy which contains at least 63% ironand includes 0-5% chromium, 10-17% boron, 4-10% silicon, and 0-5% X,wherein X is molybdenum, tungsten, or a combination of molybdenum andtungsten, and incidental impurities, all stated in weight percent, andwherein the foil is a metastable material having at least a 50% glassystructure.
 6. The brazing filler metal foil of claim 5, wherein the foilhas a thickness ranging from about 18 to 50 μm.
 7. A brazing fillermetal composition for joining objects by brazing, the brazing fillermetal composition consisting of an alloy which contains at least 63%iron and includes 0-5% chromium, 10-17% boron, 4-10% silicon, and 0-5%X, wherein X is molybdenum, tungsten, or a combination of molybdenum andtungsten, and incidental impurities, all stated in weight percent, andwherein the composition is characterized by a solidus temperature rangeof approximately 1042° C. through approximately 1174° C. and a liquidustemperature range of approximately 1148° C. through 1182° C.
 8. Thebrazing filler metal composition as recited by claim 7, said metal beingin a form of one of: a homogeneous, ductile ribbon; a powder; a foil; awire; or a preform.
 9. An iron-boron-silicon alloy, the alloy especiallyuseful for brazing stainless steel assemblies, and exhibiting effectivecorrosion resistance and minimized rates of leaching of nickel intofluids passing through either side of the stainless steel assemblies,the iron-boron-silicon alloy consisting essentially of (in weight %)about 10-17% boron, 4-10% silicon, and a balance of: iron, 0-5%chromium, and 0-5% molybdenum or tungsten.
 10. A brazed productmanufactured by brazing objects with the brazing filler metal of claim1, characterized in that the material in the objects to be brazed isstainless steel.
 11. A brazed product manufactured by brazing objectswith the brazing filler metal of claim 1, characterized in that theproduct is a shell-and-tube type heat exchanger intended for at leasttwo heat exchanging media.
 12. A brazed product manufactured by brazingobjects with the brazing filler metal of claim 1, characterized in thatthe product is a plate-plate type heat exchanger intended for at leasttwo heat-exchanging media, which comprises at least one plate packagemanufactured by brazing together a plurality of thin-walled heatexchanger plates of a stainless steel material via the brazing fillermetal at which the heat exchanger plates between themselves define plateinter-spaces intended for the heat-exchanging media.
 13. A brazedproduct manufactured by brazing objects with the brazing filler metal ofclaim 1, characterized in that the product is a plate and fin type heatexchanger intended for at least two heat-exchanging media.
 14. A methodof manufacturing a heat exchanger and an apparatus having brazed parts,comprising: juxtaposing at least two parts to define one or more jointstherebetween; supplying to said one or more joints an iron-boron-siliconbrazing filler metal alloy wherein amounts of boron and silicon arevaried to adjust liquidus and solidus temperatures of the brazing fillermetal material to desired liquidus and solidus temperatures; heatingsaid juxtaposed parts and said brazing filler metal under predeterminedconditions to melt said brazing filler metal; and cooling said brazingfiller metal to produce a brazed joint.
 15. The method as recited byclaim 14, wherein the iron-boron-silicon brazing filler metal alloyconsists essentially of a composition with the formulaFe_(a)Cr_(b)B_(c)Si_(d)X_(e), wherein X is molybdenum or tungsten, andincidental impurities, wherein the subscripts “a”, “b”, “c”, “d”, “e”are all in atom percent, and wherein “b” is between about 0 and 5, “c”is between about 10 and about 17, “d” is between about 4 and about 10,“e” is between about 0 and about 5, and a sum “a”+“b”+“c”+“d”+“e” isapproximately equal to
 100. 16. A corrosion-resistant heat exchangerwhich provides a low rate of nickel leaching, comprising at least onejoint brazed with the iron-boron-silicon brazing filler metal alloy inaccordance with the process of claim
 14. 17. The heat exchanger asrecited by claim 16, wherein said iron-boron-silicon brazing fillermetal alloy consists essentially of a composition with the formulaFe_(a)Cr_(b)B_(c)Si_(d)X_(e), wherein X is molybdenum or tungsten, andincidental impurities, wherein the subscripts “a”, “b”, “c”, “d”, “e”are all in atom percent, and wherein “b” is between about 0 and 5, “c”is between about 10 and about 17, “d” is between about 4 and about 10,“e” is between about 0 and about 5, and a sum “a”+“b”+“c”+“d”+“e” isapproximately equal to
 100. 18. A heat exchanger as recited in claim 17comprising at least two parts forming one of a plurality of brazedjoints in a brazed assembly, said heat exchanger being produced by aprocess comprising: juxtaposing said at least two parts to define one ormore joints therebetween; supplying to said one or more joints theiron-boron-silicon brazing filler metal alloy in the form of a ductile,amorphous brazing foil; heating said juxtaposed parts and said brazingfiller metal alloy to melt the brazing filler metal alloy; and coolingthe melted brazing filler metal alloy to produce the brazed assemblyhaving a brazed joint.